TextMate is an advanced text editor designed with software and web developers in mind (chimoosoft.com is maintained with it). Notable features include tabs, project level management of files, multi-file (project) search with regular expression support, text-completion, syntax highlighting for scores of languages, a command line launcher, and countless editable macros defined for each language to reduce typing and improve efficiency. Standard OS X spell checking is also included, but with a nice twist - it's smart enough to avoid flagging items such as HTML tags. A built in web preview is available and many scripting languages have the ability to run and display their output within TextMate.
In addition to your standard and expected text editor features, TextMate can do some truly amazing things. For example, when writing HTML, select a word, press a key shortcut, and TextMate will look up the best matching link for the word on Google and insert it as a link in your HTML code! If you can't remember the appropriate key combo, a floating searchable browser is available where you can quickly find the shortcut you desire. Find the large number of features overwhelming? Check out the free online "screencasts" (iTunes link) which demonstrate how to use TextMate's powerful features or browse the extensive online help.